Alan's father Pete Mahon was manager of UCD but unusually for such a situation Alan's arrival at UCD preceded his father by some 6 years. Alan joined UCD from St. Francis in 1997 and made his league debut away to Waterford United on the opening day of the 1998/99 season.

He is UCD's second longest serving player. He is third on the all-time list of league appearances for UCD. Although he scored once in the cup for UCD, Mahon had not scored a league goal after spending a decade with the club. However, Alan finally scored his first league goal in his 241st league appearance in a 3-1 defeat to Drogheda United at the Belfield Bowl . He followed up with his second ever league goal the following month .